Two-dimensional random access scanner for optical disks

ABSTRACT

A system and a method for optical access of the surface of an optical disk (or another moving media information storage device) with a very short seek time is presented. The system uses two-dimensional deflection of a light (typically laser) beam. The beam reaches a stationary lenslet array, where, at a particular moment, it intercepts a single lenslet. The lenslet focuses the beam to a spot on the information storage surface of the disk. For reading information, the beam is reflected through the lenslet. Taking advantage of the &#34;cat-eye&#34; retro-reflection principle, only one detector (or very few detectors) is needed. Since only one spot at the surface is illuminated at any given moment, this spot gets most of the laser&#39;s light, allowing sufficient power concentration when needed for writing information on the disk.

This .Iadd.application is a Reissue of application Ser. No. 08/845,288,filed Apr. 21, 1997, U.S. Pat. No. 5,764,603, which .Iaddend.applicationis a continuation of application Ser. No. 08/428,636, filed Apr. 25,1995 now abandoned.

FIELD AND BACKGROUND OF THE INVENTION

The present invention relates to an optical scanning method andapparatus for directing and moving a spot or pattern of light, typicallyfrom a laser, across a line or an area on some given surface, forexample, to access an area on a surface that moves linearly orangularly. The invention is particularly applicable to provide a rapidrandom-access optical scanner and method for optical disk data storage,and is therefore described below with respect to this application.

Optical disk data storage involves a number of somewhat conflictingrequirements, including:

1. Fast Random-Access: In such data storage, the scanned line is notaccessed in an orderly way, e.g., sequentially or predefinednon-sequentially, but rather in a random manner known only during theactual operation of the system; random-access requires that the time toreach a requested point must be as short as possible (preferably arounda millisecond or less).

2. High Resolution: That is, the size of the light spot formed by thescanning system (or, for some applications, the finest details in thepattern projected by the system) must be as small as possible, typicallyof the order-of-magnitude of the wavelength of the light; the precisionof the scanning (the precision in the location of the spot) must becomparable with the spot size.

3. Large Space-Bandwith-Product: That is, the total number of pointsaccessible by the scanning system should be large; this number roughlyequals the length of the scan line divided by the spot size (a typicalvalue should be at least in the thousands).

In addition, for some key applications, such as R/W (read/erase/write)and WORM (write-once read-many) optical computer disk systems, it isnecessary to have:

4. High Light Power Concentration: That is, it must be possible to haveenough energy delivered to a spot in a short time to affect a "write"operation.

For such R/W and WORM applications, it is desirable to focus most of theavailable laser beam power into a single spot for the "write" operation.Techniques for accomplishing this by illuminating many spotssimultaneously (parallel access). or by optically encoding the data overan area on the disc (such as holographic storage). are described in anumber of prior publications, including: Ph. Marachand. A. V.Krishnamoorthy, K. S. Urquhardt, P. Ambs, S. Esener and S. H. Lee,"Motionless Head for Parallel Readout Optical Disk," SPIE/IST Symp. onElectronic Imaging Science and Technology, February 1992, San Jose.Calif., published in Proc. SPIE, vol. 1662 (SPIE, Bellingham Wash.,1992); V. A. Ivanov, B. S. Kiselyov, A. L. Mikaelian and D. E. Okonov,"Optoelectronic Neuroprocessor Based on Holographic Disk Memory,"Optical Memory and Neural Networks, vol. 1, pp. 55-62 (1992).

The standard solution is to put the entire laser/optics assembly (ormost of it) at the end of a movable arm, just as is done today withmagnetic heads for magnetic computer disks, and earlier with mechanicalstylus pick-ups in phonograph players. However, laser/optics assembliesare heavier than magnetic disk heads. Therefore they cannot beaccelerated and moved as quickly since the inertia of an object torotation is proportional to the square of the distance of its mass fromthe center of rotation. As a result, optical disk systems are usuallyslower than magnetic disk ones.

Because light beams can be focussed and steered at a distance, it wouldbe preferable to use a movable mirror (or some other beam steeringdevice) away from the scanned surface and to keep the laser and most ofthe optics stationary. However, the combination of requirements 2 and 3above, and the physical laws of optical diffraction, would dictate arather large and heavy mirror. Such a mirror cannot be acceleratedquickly and would therefore be incompatible with the fast random-accessrequirement 1.

There is thus an urgent need for a high resolution optical scanningmethod and apparatus which provide fast random-access, comparable to orbetter than that of a non-optical magnetic disk scanning system.

BRIEF SUMMARY OF THE INVENTION

According to the present invention, there is provided a method ofdirecting an optical beam from an optical beam source to a selectedlocation on a given moving surface, comprising: interposing a staticarray of image focussing elements adjacent to the given surface on theside thereof facing the optical beam source; and steering the opticalbeam from the source to a selected one of the image focussing elementsto focus the optical beam to a selected location on the given surface.

According to further features in the preferred embodiments of theinvention described below, the static array of image focussing elementsis a two-dimensional array, and the optical beam is steered along twoaxes to a selected image focussing element.

The invention also provides apparatus operating in accordance with theabove method.

As win be more particularly described below, the novel method andapparatus enable the combined moment-of-inertia of all movable parts tobe kept as low as possible, thereby increasing speed and decreasingaccess time. In addition, it permits the focussing elements to be keptvery close to the given surface (e.g., a rotatable optical disk recordmedium) so that the physical diameter of the optical beam can be keptsmall, thereby enabling the beam to be focussed to a spot in the orderof the light wavelength.

The novel method thus enables the use of small, multi-dimensional beamsteering devices together with a static array of image focussingelements to provide rapid random-access scanning of the record medium.Two-dimensional so steering of the beam may be effected by a single beamsteering element (e.g., a mirror) driven along the two axes; preferably,however, it is effected by two beam steering elements (e.g., twomirrors) each driven along one axis. When two beam steering elements areused each offering a moderate space-bandwith-product, the total numberof separable spot positions is proportional to the product of thespace-bandwith products of the individual steering elements. Thus, usingtwo movable mirror beam steerers with even a poor (and therefore notrecommended) space-bandwith product of about 100 each enables attaininga system addressability of a few thousand locations.

The use of the static array of image focussing elements reorders thescanning pattern that is generated by the beam steering elements intoanother pattern and focusses the beam from a short distance into thenecessary spot size. Spot sizes in the order of magnitude of thewavelength of the light are thus achievable. Even though the focusedspots may not be arranged in a line, advantage is taken of the movementof the medium (e.g., rotation of an optical disk) to reach any desiredlocation.

Practically all current writable optical disk media use heating byfocussed laser beams for writing. In magneto-optical disks, writing isperformed by applying an external magnetic field and heating one spotabove the Curie point to obtain a change in the magnetization state.Such a technique is preferably used in the method and apparatusdescribed below, but it is to be understood that the invention couldalso be implemented with other optical writing techniques, such as theuse of phase-change materials.

Further features and advantages of the invention will be apparent fromthe description below.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention is herein described, by way of example only, withreference to the accompanying drawings, wherein:

FIG. 1 schematically illustrates one form of optical scanning system inaccordance with the present invention using a lenslet array for theimage focussing elements;

FIG. 2 is an enlarged sectional view of a portion of the system of FIG.1;

FIG. 3 is a top plan view of one of many possible lenslet arrays thatmay be used in the system of FIG. 1;

FIG. 4 is an enlarged, fragmentary, cross-sectional view of a modifiedlenslet array;

FIG. 5 illustrates a blocking layer that may be used in the lensletarray;

FIG. 6 illustrates a modification in the system of FIG. 1 to change theeffective location of the beam steering mirrors;

FIG. 7 illustrates another construction of blocking layer that may beused with another lenslet array;

FIG. 8 illustrates another modification in the system of FIG. 1 toenable detection of tracking and focus errors;

FIG. 9 illustrates one form of planar optical member which may be usedin order to provide a more compact system;

and FIG. 10 illustrates another form of planar optical member that maybe used to provide a more compact system.

It is to be understood that the above figures are not to scale. Althoughin most cases the optical system is depicted as if it is located on topof the record medium, the optical system could be located below therecord medium. The number of focussing elements in the array of anactual system will in most cases differ from the illustrations in thedrawings.

DESCRIPTION OF PREFERRED EMBODIMENTS

FIG. 1 illustrates one form of optical scanning apparatus constructed inaccordance with the invention, wherein a laser beam from laser 2 iscontrolled to scan any selected location on a record medium, such as anoptical disk 3 on a rotating holder 4. The laser beam exiting from thelaser 2 first passes through beam shaping optics, schematicallyindicated at 5, to a beam steering system, generally designated 6, whichsteers the beam along two axes to any selected location. Thus, the beamsteering system includes a mirror 7 reciprocated by a drive M₁ along oneaxis, and a second mirror 8 reciprocated by a drive M₂ along the otheraxis.

A static array of image focussing elements, generally designated 10, isprovided adjacent to the optical disc 3 on the side thereof facing theoptical beam source 2 and the beam steering system 6. The static array10 is a two-dimensional array of refractive lenslets effective to focusthe image of a beam entering each lenslet to a small spot on the opticaldisc 3.

The illustrated system further includes a beam splitter 11 aligned withbeam steerer mirror 8. Beam splitter 11 is a static device and iseffective to pass the beam from mirror 8 to the static array 10. Some ofthe light is reflected back from focussed spots on the optical disc viaarray 10 to the beam splitter 11, which is effective to relect thelatter light to a static detector assembly 12.

FIG. 2 is a sectional view through one row of lenslets in the array 10and illustrates the paths of the various beams from the steering mirror8 of the beam steering system 6. This figure schematically shows thepaths of the laser beam 20 as reflected by mirror 8 in three differentpositions of the mirror. Thus, in one position of the mirror, the beam(as, defined by the two single-arrow lines 21) will be focussed to apoint P₁ on the optical disk 3; in a second position of the mirror, thebeam (as defined by the two double-arrow lines 22) will be focussed to apoint P₂ on the optical disk 3; and in a third position, the beam (asdefined by the two triple-arrow lines 23) will be focussed to a thirdpoint P₃ on the optical disk 3. It is to be noted that more than asingle point can be accessed through the same lenslet.

FIG. 3 is a top view illustrating one form of lenslet array 10 that maybe used. Reference numeral 31 at the right side of FIG. 2 illustrates anindividual lenslet, and reference numeral 32 depicts the locus of allpoints that can be accessed through that lenslet. This scanned patternis a collection of spots of light that may be focussed by the lenslet onthe surface of the optical disk 3. Arrow 33 of the left side of FIG. 3indicates the direction of movement of the optical disk with respect tothe lenslet array 10. FIG. 3 illustrates the scanned patterns for eachof the lenslets. FIG. 7, described below, illustrates another lensletarrangement that may be used.

It is to be noted that the lenslets in the array illustrated in FIG. 3are not arranged in regular vertical columns, but rather are somewhatstaggered. A conventional, non-staggered lenslet array can be used if itis slightly rotated so that the rows of the lenslets are notperpendicular to the direction of movement of the disk surface.

The skewing of the lines of the lenslets in the array, as shown in FIG.3 (also in FIG. 7 to be described below), or a slight rotation of thecartesian lenslet array, is an important feature of the illustratedsystem. This allows filling in points that cannot be accessed by asingle row of lenslets and thus enables access to all desirable pointson the moving record medium.

FIG. 4 illustrates a modified construction in the lenslet array. In thismodified lenslet array, therein designated 40, the individual lenslets41 are not adjacent to each other, but rather are spaced apart bysections 42. A light blocking layer or mask 43 is applied over the uppersurface of the lenslet array 40 and is formed with openings 44 alignedwith the individual lenslets 41. Thus, mask 43 permits the optical beamto pass only to the individual lenslets 41, and prevents light fromreaching the sections 42 between the individual lenslets.

FIG. 4 illustrates four light beams 45, 46, 47, 48, corresponding tofour positions of the beam steering system 6 (FIG. 1). Beams 45 and 46are focussed by the left lenslet 41 into two distinct spots on thesurface of the optical disk 3; beam 47 is blocked by the mask 43; andbeam 48 is focussed by the right lenslet 41 onto the surface of theoptical disk 3.

FIG. 5 illustrates the construction of the blocking mask 43, whereineach of the openings 44 has a diameter d_(b). Their centers are spacedalong the X-axis and Y-axis by the distances P_(x) and P_(y),respectively. The optical beam diameter in this plane is d₁, and thebeam location is defined as x₁, y₁.

When the blocking layer 43 is very close to the lenslet array 40, thelaser light cannot reach two lenslets at the same time if:

    d.sub.1 <P.sub.x -d.sub.b                                  (1)

    d.sub.1 <P.sub.y -d.sub.b

To obtain a spot size (d_(S)) of about 1.2λ (wherein "λ" is thewavelength of the light), it is necessary to have a converging beam witha cone angle of roughly 60° so that the focal length for each of thelenslets must be F=d_(b). It can be shown that the number of tracks K₁that can be spanned through a single lenslet 41 is as follows: ##EQU1##wherein Z_(c) is the distance between the closest steering mirror (8 inFIGS. 1 and 2) and the surface of the optical disk 3.

For example, let us assume λ=0.9×10⁻³ mm, lenslet array pitch p_(x)=p_(y) =1 mm, and blocking layer apertures of a diameter d_(b) =0.3 mm.To satisfy eq. (1) we select d₁ =0.7 mm. For Z_(c) =13 mm we get K₁ =10.

As evident from this example, it is sometimes desirable to have a Z_(c)that is smaller than can be easily realizable physically.

FIG. 6 illustrates the use of a large negative lens 50 to make thelenslet array see the virtual image VI of the beam steering mirror (8),at height Z'_(c), which is smaller than Z_(c). With the configuration ofFIG. 6. Z'_(c) is calculated using the standing imaging formula for thenegative lens, and 1Z'_(c) 1 is then used instead of Z_(c) in eq. (2).

Instead of using a large negative lens 50, this lens may also be apositive lens to provide Z'_(c) <0 such that |Z_(c) '|<Z_(c). This lensmay also be a cylindrical lens or a torus-shaped lens.

The blocking layer 43 may have non-circular openings 44, such asrectangular. Also, the optical beam may also have a shape other thancircular at the plane of the lenslet array.

Further, it may be desirable to arrange the lenslets according to anon-cartesian array. In such case, the blocking mask would be similarlyformed with a non-cartesian array of openings, as shown at 44' in FIG.7.

For reading (or writing) data from optical disks, it is necessary to beable to locate and track the requested data precisely and in focus.Since it is not practical to manufacture and/or position optical diskswithin the tolerances (centering and surface flatness) necessary toachieve this precision in an open loop system. FIG. 8 illustrates afeedback-based tracking and focussing system which may be used.

Schemes using three beams for tracking error detection and astigmaticfocussing error detection are already in use with conventional (head atthe end of arm) optical disk drives. FIG. 8 illustrates how these wellknown methods can be implemented in the above-described system. Anexample of a paper describing the prior art with respect totracking/focus error detection in conventional optical disk drives is:W-H Lee. "Holographic Optical Head for Compact Disk Applications,"Optical Engineering. vol. 28. pp. 650-653 (1989).

The system in FIG. 8 includes a phase grating 54 next to the beam shaper5. Phase grating 54 splits the single beam into three differentdiffraction orders (-1,0, and +1). A low power cylindrical lens 51 isadded in front of the static detector assembly (12, FIG. 1). Lens 51generates and introduces a small amount of astigmatism into the image ofthe laser spot detected by the detector assembly.

Grating 54 thus splits the laser beam into three orders. The strongerzero order is used to read and write data, whereas the two ±1 orders areused to detect tracking errors. They create two extra spots on the disksurface which are normally slightly off track. When there is a trackingerror, one of these side spots reflects more light than the other, sothat the direction of the error can be detected by the detector assembly12 and used for controlling the tracking system in a feedback loopincluding tracking control unit 52.

An alternative arrangement to grating 50 would be to provide a largegrating (not shown) placed on top of the lenslet array 10.

Cylindrical lens 51 is used to provide a focus error signal. Byintroducing some astigmatism into the image of the spot, the imagebecomes elliptical when the system is off focus. The direction of themajor axis of this elliptical spot image indicates the sign of theerror. This focussing error may thus also be used to correct focus in afeedback loop from detector assembly 12 to a focus control unit 53.

An alternative arrangement would be to introduce some intentionalastigmatism through the individual lenslets in the array, or in thelarge lens (50, FIG. 6).

The system may also be made more compact by using planar optical memberswhich allow trapping and manipulating light in a small volume. FIGS. 9and 10 illustrate two examples of planar optical members that may beused for this purpose.

In FIG. 9 the planar optical member is in the form of a transparent slab60 having one surface 61 which is slanted with respect to its opposedsurface 62. Slab 60 is placed with its surface 62 over, in closeproximity to or in contact with, the static array 10 of the lensletsoverlying the optical disk 3.

The optical beam 63 from the beam steering system (6, 45 FIG. 1) isreflected by a removable reflector 64 (which replaces mirrors 7 and 5 ofFIG. 1) into the edge of the planar optical member 60. The convergingsurfaces 61, 62 of member 60 produce multiple reflections of the opticalbeam at decreasing angles to the normal of its surface 62 facing thelenslet array 10. Surface 62 may be spaced from the upper surface of thelenslet array 10 by space, shown at 65, in the form of a simple air gap,or by an interferometric multi-layer coating applied for example byvacuum deposition or by optical holographic recording.

The laser beam 63, after it has already passed through beam shapingoptics, is deflected by the beam steering mirror 64 (or two mirrors,only one of which is shown) to enter the slab 60 from the side. Theupper surface 61 of the slab reflects back all beams, either by totalinternal reflection or by the help of a reflective layer. Since thissurface is sloped, the angle of the reflected beam relative to the planeof the bottom surface 62 changes after each reflection; i.e., its anglewith respect to the normal to surface 62 decreases. As a beam reachesthe bottom surface of the slab 60, it will be either reflected bysurface 62 back towards surface 61, or transmitted through surface 62.This is controlled by layer 65 which, as indicated earlier, may be asimple air gap or an interferometric multi-layer coating. In eithercase, beams at relatively large angles to the normal to surface 10 arereflected back into the slab, whereas beams of small angles to thenormal to surface 62 pass through. As a beam is reflected back and forthbetween the two surfaces 61, 62, the angle of its incidence changesgradually until, at one place, it can pass through surface 62.

The location where a beam passes through surface 62 and layer 65 isdictated by the original direction of the beam, as dictated by the beamsteering mirror 64. Once a beam gets through, it is focussed by therespective image focussing element in the static array 10.

FIG. 10 illustrates an arrangement including a planar optical member 70in the form of a transparent slab having a rectangular cross sectionwith parallel top and bottom surfaces. In this case, the top layer 71 isholographic and reflects light to the bottom layer 72 which contains, oroverlies, the static array 10 of the image focussing elements. Thus, thelaser beam 73 is directed to the beam steering side mirror 74 whichreflects the beam into the edge of the planar optical member 70. Thebeam exits at a preselected location, as determined by the beam steeringmirror 74, to be focussed by the respective focussing element of thestatic array 10 onto the optical disk 3.

It will thus be seen that the two arrangements illustrated in FIGS. 9and 10 act similarly to the previously described arrangements exceptthat the planar optical member (60 in FIG. 9 and 70 in FIG. 10) act tocompress the space that exists between the beam steering elements andthe static array of image focussing elements.

Other schemes to compress space are also possible, including the use ofone or more mirrors to fold the optical path, or the use of planaroptical elements of different shape and/or design than the ones depictedin FIGS. 9 and 10. Such planar optical elements may use a curved, or amulti-faceted surface or surfaces.

In all the previously described embodiments, the static array of imagefocussing elements is described as an array of refractive lenslets. Itwill be appreciated, however, that the image focussing elements in anyone of the above-described embodiments could be an array of diffractivelenses, such as an array of holograms or a multi-facet mirror. Theoptical beam in the above-described embodiment is from a diode laser,but other optical beams could be used. Also, while the record medium isshown as being a rotatable disk, it will be appreciated that otherrecord media could be used, such as a tape, or a drum. Further, whilethe above described embodiments relate to optical recording and readingof data, the invention can also be used for computer graphics and otherapplications.

Many other variations, modifications and applications of the inventionwill be apparent.

I claim:
 1. A method of directing an optical beam from an optical beamsource to a selected storage location spot on the surface of a movingoptical storage medium, comprising:interposing a static two-dimensionalarray of image focussing elements adjacent to the surface between thesurface and the optical beam source; and steering the optical beam fromthe source to a selected single one of said image focussing elements inthe two-dimensional array to focus the optical beam to a single selectedstorage location spot on the surface.
 2. A method in accordance withclaim 1, wherein the storage location spot to which the optical beam isfocussed has a size in the order of magnitude of the light wavelength ofthe optical beam.
 3. The method according to claim 1, wherein theoptical storage medium is a rotatable optical data disk.
 4. The methodaccording to claim 1, wherein said static two-dimensional array of imagefocussing elements is an array of refractive lenslets.
 5. The methodaccording to claim 1, wherein said static two-dimensional array of imagefocussing elements is an array of diffractive lenses.
 6. The methodaccording to claim 1, further comprising interposing an apertured maskbetween the array of image focussing elements and the optical beamsource to prevent the optical beam from reaching two image focussingelements at the same time.
 7. The method according to claim 1, furtherincluding the step of reflecting the optical beam from the focussed spotvia an image focussing element to a static detector assembly.
 8. Themethod according to claim 7, wherein the reflected optical beam isreflected via a beam splitter to the static detector assembly.
 9. Themethod according to claim 7, further including introducing someastigmatism into the optical beam reflected from the selected imagefocussing element and using said introduced astigmatism for detectingand correcting focussing errors.
 10. The method according to claim 1,further including providing a lens adjacent the image focussing elementsbetween the image focussing elements and the optical beam source toproduce a virtual image of the optical beam close to the image focussingelements.
 11. The method according to claim 1, further includingdisposing a planar optical member over the array of image focussingelements, wherein said steering step comprises steering the optical beaminto an edge of the planar optical member.
 12. The method according toclaim 1, further including passing said optical beam through a gratingbefore the optical beam reaches said static array of image focussingelements to split the beam into a zero order straddled by twoweaker-orders, and using said two weaker orders for detecting andcorrecting tracking errors.
 13. Apparatus for directing an optical beamfrom an optical beam source to a selected storage location spot on thesurface of a moving optical storage medium, comprising:a statictwo-dimensional array of image focussing elements located adjacent tothe surface between the surface and the optical beam source; and meansfor steering the optical beam from the source to a selected single oneof said image focussing elements in said two-dimensional array to focusthe optical beam to a single selected storage location spot on thesurface.
 14. An apparatus in accordance with claim 13, wherein thestorage location spot to which the optical beam is focussed has a sizein the order of magnitude of the light wavelength of the optical beam.15. The apparatus according to claim 13, wherein the optical storagemedium is a rotatable optical disk, and further including a holder forthe rotatable optical disk.
 16. The apparatus according to claim 13,wherein said static two-dimensional array of image focussing elements isan array of refractive lenslets.
 17. The apparatus according to claim13, wherein said static two-dimensional array of image focussingelements is an array of diffractive lenses in the form of a hologram.18. The apparatus according to claim 13, further including an aperturedmask interposed between said two-dimensional array of image focussingelements and the optical beam source, which mask prevents the opticalbeam from reaching two image focussing elements at the same time. 19.The apparatus according to claim 13, further including means fordetecting the optical beam after reflection from a focussed spot via oneof said image focussing element.
 20. The apparatus according to claim19, further including a beam splitter located to pass the optical beamfrom the optical beam source to the array of image focussing elements,and to reflect the optical beam reflected from the selected imagefocussing element to said means for detecting.
 21. The apparatusaccording to claim 20, further including a lens between said beamsplitter and said means for detecting, said lens designed to introducesome astigmatism into the beam, and a control system for detecting andcorrecting focussing of the beam in response to the output of said lens.22. The apparatus according to claim 13, further including a negativelens adjacent to the image focussing elements between said elements andthe optical beam source, said negative lens being designed to produce avirtual image of the optical beam close to the image focussing elements.23. The apparatus according to claim 13, further including a planaroptical member overlying the array of image focussing elements, saidmeans for steering steering the optical beam into an edge of said planaroptical member.
 24. The apparatus according to claim 23, wherein saidplanar optical member is a slab having one surface which is slanted withrespect to an opposed surface to thereby produce multiple-reflections ofthe optical beam steered into the edge of said planar optical member atdecreasing angles to the normal of said opposed surface until theoptical beam exits from said member at a selected location thereof to aselected one of said image focussing elements.
 25. The apparatusaccording to claim 23, wherein said planar optical member includes aholographic layer on the side thereof opposite to that facing said imagefocussing elements, to reflect the light to a selected one of said imagefocussing elements.
 26. The apparatus according to claim 13, furtherincluding a grating located upstream of said static array of imagefocussing elements to split the beam into a zero order straddled by twoweaker±orders, and a control system for detecting and correctingtracking errors in response to said two weaker orders.
 27. In a methodfor reading from and writing to selected storage location spots on anoptical storage medium using a focussed optical beam the improvementwherein the optical beam is directed from an optical beam source to aselected storage location spot on the optical storage medium by stepscomprising:interposing a static two-dimensional array of image focussingelements adjacent to the optical storage medium, between the opticalstorage medium and the optical beam source; and steering the opticalbeam from the optical beam source to a selected single one of said imagefocussing elements in the two-dimensional array to focus the opticalbeam to a single selected storage location spot on the optical storagemedium.